[Mesa-dev] [Mesa-stable] [PATCH] configure.ac: fix test for SSE4.1 assembler support

[Matt Turner]

On Sun, Dec 13, 2015 at 5:23 AM, Oded Gabbay <oded.gabbay at gmail.com> wrote:
> On Sun, Dec 13, 2015 at 11:56 AM, Jonathan Gray <jsg at jsg.id.au> wrote:
>> On Sat, Dec 12, 2015 at 06:41:56PM +0000, Emil Velikov wrote:
>>> On 10 December 2015 at 08:42, Oded Gabbay <oded.gabbay at gmail.com> wrote:
>>> > On Wed, Dec 9, 2015 at 8:30 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>> >> On Tue, Dec 8, 2015 at 9:37 PM, Jonathan Gray <jsg at jsg.id.au> wrote:
>>> >>> Change the __m128i variables to be volatile so gcc 4.9 won't optimise
>>> >>> all of them out with -O1 or greater.  The _mm_set1_epi32/pinsrd calls
>>> >>> still get optimised out but now there is at least one SSE4.1 instruction
>>> >>> generated via _mm_max_epu32/pmaxud.  When all of the sse4.1 instructions
>>> >>> got optimised out the configure test would incorrectly pass when the
>>> >>> compiler supported the intrinsics and the assembler didn't support the
>>> >>> instructions.
>>> >>>
>>> >>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91806
>>> >>> Signed-off-by: Jonathan Gray <jsg at jsg.id.au>
>>> >>> Cc: "11.0 11.1" <mesa-stable at lists.freedesktop.org>
>>> >>> ---
>>> >>>  configure.ac | 2 +-
>>> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>> >>>
>>> >>> diff --git a/configure.ac b/configure.ac
>>> >>> index 260934d..1d82e47 100644
>>> >>> --- a/configure.ac
>>> >>> +++ b/configure.ac
>>> >>> @@ -384,7 +384,7 @@ CFLAGS="$SSE41_CFLAGS $CFLAGS"
>>> >>>  AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
>>> >>>  #include <smmintrin.h>
>>> >>>  int main () {
>>> >>> -    __m128i a = _mm_set1_epi32 (0), b = _mm_set1_epi32 (0), c;
>>> >>> +    volatile __m128i a = _mm_set1_epi32 (0), b = _mm_set1_epi32 (0), c;
>>> >>>      c = _mm_max_epu32(a, b);
>>> >>>      return 0;
>>> >>
>>> >> I would have extracted an int from the result of _mm_max_epu32 and
>>> >> returned that instead of 0.
>>> >
>>> > Instead of the volatile I assume ?
>>> >
>>> Precisely. If anyone wants to follow on Matt's suggestion we can pick
>>> that one as well. I'd like to get a patch for the next stable releases
>>> (next Friday for 11.0.x and just after new year for 11.1.1) so I'll
>>> take whatever's around :-)
>>>
>>> -Emil
>>
>> I avoided that as I wasn't sure if there was a case where autoconf
>> cared about the return code.  If someone wants to create a new diff
>> feel free, I have limited connectivity till the middle of next week.
>
> So I'm not a huge SSE expert, but I tried doing this (remove volatile
> and return _mm_cvtsi128_si32 of c):
>
> ------------------------
> #include <mmintrin.h>
> #include <xmmintrin.h>
> #include <emmintrin.h>
>
> int main () {
>     __m128i a = _mm_set1_epi32 (0), b = _mm_set1_epi32 (0), c;
>     c = _mm_xor_si128 (a, b);
>     return _mm_cvtsi128_si32(c);
> }
> -------------------------
>
> When compiling with "gcc -O1 -msse2", gcc is 4.8.5 (from RHEL 7.2), I got:
>
> ---------------------
> main:
> .LFB521:
> .cfi_startproc
> movl $0, %eax
> ret
> .cfi_endproc
> -------------------
>
> So unless I misunderstood matt's suggestion, I think we *have* to use
> the volatile as it forces the compiler to produce pxor and movdqa
> assembly commands.

Since all the arguments to the intrinsics are constants, GCC is
constant-evaluating them.

I expect all you'd need to do is pass some global variables to the
intrinsics or similar.